Bath for the electrochemically depositing highly lustrous white rhodium coatings and whitening agent for the same

ABSTRACT

The invention concerns a bath for the electrochemical deposition of high-gloss white rhodium coatings and a whitening agent for the same.  
     The brightness or degree of whiteness of the deposited coatings is significantly increased by means of compounds having the formula I  
     R—SO m —H  (I)  
     wherein  
     m is the numbers 3 or 4 and  
     R is a straight-chain or branched or cyclic alkyl group having up to 20 C atoms,  
     as whitening agents. The thickness of the coating that can be deposited without a bloom is also significantly increased.

[0001] The invention concerns an electroplating bath for the depositionof high-gloss white rhodium coatings and a whitening agent for the same.

[0002] Rhodium coatings are deposited onto silver e.g. as a tarnishpreventive and should exhibit high gloss and as bright, white a colouras possible, similar to that of silver.

[0003] Commercial electrolytes are composed on the basis of rhodiumsulfate, phosphate or sulfamate, sulfuric acid, phosphoric acid, alkanesulfonic acid or amidosulfonic acid.

[0004] In order to be able to deposit bloom-free coatings from suchbaths, they conventionally contain certain organic compounds asbrightening additives. Typical brightening additives that are verycommonly used, as described in EP 0 056 590, for example, are thecompounds pyridine-3-sulfonic acid and naphthaline trisulfonic acid. Toprevent hydrogen from adhering to the cathode, a wetting agent and/or aphosphonic acid can additionally be contained in such a bath.

[0005] A disadvantage of the existing systems lies in the fact that thevery bright, white colour of silver is not achieved and that as the filmthickness increases deposition occurs with a greater bloom.

[0006] The object of the invention was therefore to achieve animprovement in such rhodium baths in the respect that the depositedcoatings are significantly whiter and the brightness or degree ofwhiteness is markedly closer to silver. The thickness of film that canbe deposited without a bloom should also be increased.

[0007] Surprisingly it has been found that this can be achieved if atleast one compound having the general formula I

R—SO_(m)—H  (I)

[0008] wherein

[0009] m is the numbers 3 or 4 and

[0010] R is a straight-chain or branched or cyclic alkyl group having upto 20 C atoms

[0011] is added as whitening agent to such baths for the deposition ofrhodium coatings.

[0012] The invention thus provides a bath for the electrochemicaldeposition of high-gloss white rhodium coatings, containing rhodium indissolved form optionally with an organic compound as brighteningadditive, characterised in that the bath contains as whitening agent atleast one compound having the general formula

R—SO_(m)—H  (I)

[0013] wherein

[0014] m is the numbers 3 or 4 and

[0015] R is a straight-chain or branched or cyclic alkyl group having upto 20 C atoms.

[0016] The whitening agents according to formula I are selectedcompounds from the class of alkyl sulfates or alkyl sulfonates. Informula I R denotes a straight-chain or branched or cyclic alkyl grouphaving up to 20 C atoms.

[0017] The compounds having formula I are known per se and readilyavailable.

[0018] These compounds are adequately water-soluble and compatible withthe electroplating bath. The compounds have surfactant properties,whereby the corresponding action is reduced if the total number of Catoms is less than 4 and the solubility is generally no longer adequateif the total number of C atoms is greater than 20.

[0019] Preferred brightening additives are compounds having formula I inwhich R stands for straight-chain or branched or cyclic alkyl groupshaving 5 to 12 C atoms and in particular for branched alkyl groupshaving 6 to 10 C atoms.

[0020] Branched compounds are suitable because of their only slightlypronounced foaming tendency, particularly in processes and equipment inwhich severe foaming would be disruptive, e.g. in air-operatedelectrolytes, in drum processing, in high-speed deposition equipment(spraying equipment) and in selective deposition equipment, such as e.g.dip coating cells.

[0021] Typical whitening agents according to the invention are

[0022] hexyl sulfate

[0023] hexyl sulfonate

[0024] 2-ethylhexyl sulfate

[0025] heptyl sulfate

[0026] octyl sulfate

[0027] octyl sulfonate

[0028] decyl sulfate

[0029] decyl sulfonate

[0030] dodecyl sulfate

[0031] 7-ethyl-2-methyl-4-undecanol sulfate

[0032] cyclohexyl sulfate

[0033] and isomers thereof.

[0034] These compounds can also be in the form of their salts.

[0035] The whitening agent according to the invention is convenientlyused in a concentration range of 0.01 to 10 g/l in baths for theelectrochemical deposition of rhodium coatings. Baths according to theinvention containing the whitening agent according to formula I in aconcentration of 0.1 to 6 g/l are particularly advantageous.

[0036] The brightness or degree of whiteness of the deposited coatingsis unexpectedly significantly increased by the use according to theinvention of the compounds the compounds having formula I as whiteningagents in electroplating rhodium baths of otherwise conventionalcomposition. The maximum coating thickness at which high-glossdeposition coatings can still be obtained is likewise significantlyincreased.

[0037] Many common and commercial electroplating rhodium baths, to whichthe corresponding quantity of compound having formula I is added, can beused as a basis for the production of the rhodium baths according to theinvention.

[0038] The electroplating rhodium baths according to the inventiontypically contain approximately 0.1-20 g/l rhodium as rhodium sulfate,phosphate, alkane sulfonate or sulfamate, 10-200 g/l sulfuric acid,phosphoric acid, amidosulfuric acid or mixtures of these acids, 0-5 g/lpyridine-3-sulfonic acid as brightening agent, 0.01-2 g/l wetting agent0.1-10 g/l compound having formula I as whitening agent according to theinvention.

[0039] The baths are conventionally operated at current densities of0.5-5 A/dm² (frame operation) and temperatures of up to 60° C.

[0040] The invention is clarified by means of the following examples:

[0041] Note: All brightness values (L*) were measured in the CIE-L*a*b*system (1976) using an SP68S colorimeter from X-Rite.

EXAMPLE 1

[0042] Coatings having a current efficiency of 12.8% and a brightnessL*=88.7 are achieved on pre-nickel plated sheets measuring 25×40 mm(test set-up: 1 litre beaker, platinum-coated titanium anode, nomovement, at a current density of 1 A/dm² in a rhodium electrolyte with

[0043] 2 g/l rhodium in the form of rhodium phosphate

[0044] 32 g/l sulfuric acid

[0045] 0.2 g/l pyridine-3-sulfonic acid

[0046] pH<1

[0047] temperature 40° C.

[0048] The coatings produced from this electrolyte can be deposited upto a maximum thickness of 0.3 μm without bloom.

[0049] The addition of 2 g/l octyl sulfonate reduces the currentefficiency at 1 A/dm² only negligibly to 12.1% (1 A/dm²), but increasesthe brightness to L*=89.8. The coatings produced from this bath can nowbe deposited in a thickness of up to 0.5 μm without bloom.

EXAMPLE 2

[0050] Coatings having a current efficiency of 37.8% and a brightnessL*=87.2 are achieved on pre-nickel plated sheets measuring 25×40 mm(test set-up: 1 litre beaker, platinum-coated titanium anode, bathmovement 200 rpm by means of a 60 mm magnetic stirring rod, productmovement 5 cm/s, at a current density of 1 A/dm² in a rhodiumelectrolyte with

[0051] 2 g/l rhodium in the form of rhodium sulfate

[0052] 30 g/l sulfuric acid

[0053] 0.2 g/l pyridine-3-sulfonic acid

[0054] 20 mg/l wetting agent (fluorosurfactant)

[0055] pH<1

[0056] temperature 40° C.

[0057] At 2 A/dm² the current efficiency is still 26.3% and thebrightness reaches a value of L*=86.3. The coatings produced from thiselectrolyte can be deposited up to a maximum thickness of 0.3 μm withoutbloom.

[0058] The addition of 2 g/l 2-ethylhexyl sulfate reduces the currentefficiency only negligibly to 37.1% (1 A/dm²) and 26.0% (2 A/dm²) butincreases the brightness to L*=89.5 (1 A/dm²) and L*=90.0 (2 A/dm²).Furthermore, coatings can be deposited from this bath in a thickness of0.7 μm without bloom. The electrolyte is very low foaming and istherefore supremely suitable for drum operation.

1. Bath for the electrochemical deposition of high-gloss white rhodiumcoatings, containing rhodium in dissolved form optionally with anorganic compound as brightening additive, characterised in that the bathcontains as whitening agent at least one compound having the generalformula R—SO_(m)—H  (I) wherein m is the numbers 3 or 4 and R is astraight-chain or branched or cyclic alkyl group having up to 20 Catoms.
 2. Electroplating bath according to claim 1, characterised inthat it contains as whitening agent at least one compound having formulaI, wherein m is the numbers 3 or 4 and R is a straight-chain or branchedalkyl group having 5 to 12 C atoms, preferably a branched alkyl grouphaving 6 to 10 C atoms.
 3. Electroplating bath according to claim 1 or2, characterised in that as whitening agent it contains the compoundshexyl sulfate, hexyl sulfonate, 2-ethylhexyl sulfate, heptyl sulfate,octyl sulfate, octyl sulfonate, decyl sulfate, decyl sulfonate, dodecylsulfate, 7-ethyl-2-methyl-4-undecanol sulfate, cyclohexyl sulfate orisomers thereof.
 4. Electroplating bath according to claims 1 to 3,characterised in that it contains 0.01 to 10 g/l, preferably 0.1 to 6g/l, of compound having formula I.
 5. Use of compounds having thegeneral formula R—SO_(m)—H  (I) wherein m is the numbers 3 or 4 and R isa straight-chain or branched or cyclic alkyl group having up to 20 Catoms, as whitening agents in baths for the electrochemical depositionof glossy rhodium coatings, containing rhodium in dissolved form, andoptionally an organic compound as brightening additive and a wettingagent.